Assertions are recognized as a powerful tool for automatic run time detection of software errors. However, existing testing methods do not use assertions to generate test cases. We present a novel approach of automated test data generation in which assertions are used to generate test cases. In this approach the goal is to identify test cases on which an assertion is violated. If such a test is found then this test uncovers an error in the program. The problem of finding program input on which an assertion is violated may be reduced to the problem of finding program input on which a selected statement is executed. As a result, the existing methods of automated test data generation for white box testing may be used to generate tests to violate assertions. The experiments have shown that this approach may significantly improve the chances of finding software errors as compared to the existing methods of test generation.